Page 227 - Computational Retinal Image Analysis

224 CHAPTER 12 Diabetic retinopathy and maculopathy lesions

abnormal development and angiogenesis of new vessels are the main representative
characteristics of the disease: new blood vessels can grow in different areas of retinal
surface or at the surface of the optic disc. The former are defined as New Vessels
Elsewhere (NVE), the latter as New Vessels on Disc (NVD). These abnormal new
vessels can easily bleed and are considered as the major leading cause of DR-related
blindness.

2 The clinical impact of DR and maculopathy lesions
DR should be detected in the early stages so that treatment can be initiated before
the development of vision-threatening retinopathy (ETDRS report number 9 1991).
Regular retinal screening is appropriate because DR constitutes a major health problem,
which is frequent among diabetic patients, can be detected and treated, and also because
screening is cost-effective [3–10]. Screening for DR includes the measurement of best-
corrected visual acuity and fundus photography. The photographic procedure has been
standardized, and generally includes seven-field photography with 30-degree images
in the United States, and two 60-degree fundus photographs (FPs) in Europe, one
centered on the foveal area and another centered on the optic disc [11, 12]. The grading
of DR in these FPs is performed by trained graders, and the degree of retinopathy
is compared with a set of standard images representing the range of severity of the
disease. This results in a semiquantitative grading of retinopathy. After each screening
examination, the recommended interval to the following examination is assessed on the
basis of rules designed so that progression to a vision-threatening condition is detected
even in patients with the fastest disease progression [13, 14]. The screening interval
is mainly defined on the basis of retinopathy grade, but diabetes type and diabetes
duration are also included in the recommendation. Therefore, there is a lot of interest
in the identification of early signs of the disease. MAs are the most frequent type of
lesions to appear on the retina and the first symptom of DR [15]. It has been found that
MAs are so important for the diagnosis that the presence of even one or two in an eye
should be considered seriously [16]. Small solitary MAs can be potentially missed or
overlooked by DR graders and screeners who are involved in reviewing thousands of
screening images on regular basis, hence automated detection of DR lesions can help
reduce human error and alleviate work burden of the retinopathy graders.

3 Type of lesions/clinical features
As alluded to earlier, DR has no early warning signs, nevertheless initial changes of
DR manifest as a direct consequence of retinal capillary damage. In order, abnormal
changes of the large arterioles and venules may be seen. Morphological alterations,
which are shown in Fig. 1, may reflect an ongoing series of pathological changes,
including previous retinal damage.
Donnellyand Horton [17] and Van Bijsterveld [18] explain that while DR